U.S. patent application number 11/046155 was filed with the patent office on 2005-07-28 for variable-frequency high frequency filter.
Invention is credited to Asamura, Fumio.
Application Number | 20050162241 11/046155 |
Document ID | / |
Family ID | 34797816 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050162241 |
Kind Code |
A1 |
Asamura, Fumio |
July 28, 2005 |
Variable-frequency high frequency filter
Abstract
A frequency-variable high frequency filter comprises: a
substrate; a ground conductor having an opening provided on one
principal surface of the substrate; a center conductor provided in
the opening to make up a coplanar line resonator with the substrate
and the ground conductor; an input line and an output line provided
on the other principal surface of the substrate and
electromagnetically coupled with the center conductor; and a
variable reactance element. The center conductor is divided into
two conductor section at the position of the null point of the
voltage displacement of a standing wave created in the coplanar
line resonator so that the two conductor sections are separated in
the longitudinal direction of the center conductor. The variable
reactance element is inserted in the center conductor to connect
the two conductor sections of the center conductor.
Inventors: |
Asamura, Fumio; (Saitama,
JP) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
34797816 |
Appl. No.: |
11/046155 |
Filed: |
January 28, 2005 |
Current U.S.
Class: |
333/205 |
Current CPC
Class: |
H01P 1/2013
20130101 |
Class at
Publication: |
333/205 |
International
Class: |
H01P 001/203 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 2004 |
JP |
2004-020503 |
Nov 15, 2004 |
JP |
2004-330532 |
Claims
What is claimed is:
1. A high frequency filter comprising: a substrate, a ground
conductor having an opening provided on one principal surface of
said substrate, a center conductor provided on said one principal
surface of said substrate in said opening and making up a coplanar
line resonator of a coplanar structure together with said substrate
and said ground conductor, an input line and an output line
provided on the other principal surface of said substrate and
adapted to electromagnetically couple with said center conductor,
and a variable reactance element inserted in said center conductor
at the position of a null point in voltage displacement of a
standing wave created in the center conductor operating as a
resonator, wherein said center conductor is divided into a first
conductor section and a second conductor section at the inserted
position of said variable reactance element so as to separate the
first and second conductor sections in a longitudinal direction of
said center conductor, said variable reactance element being
connected at one end to said first conductor section and at the
other end to said second conductor section.
2. The filter according to claim 1, further comprising means for
applying a control voltage to said variable reactance element.
3. The filter according to claim 2, wherein said variable reactance
element comprises a variable capacitance diode.
4. The filter according to claim 1, wherein at least one of said
input line and said output line has a portion that traverses said
center conductor.
5. The filter according to claim 1, wherein both of said input line
and said output line have respective portions that traverse said
center conductor.
6. The filter according to claim 5, wherein distance from a
position where said input line traverses said center conductor to
one end of said center conductor equals distance from a position
where said output line traverses said center conductor to the other
end of said center conductor.
7. The filter according to claim 5, wherein distance from a
position where said input line traverses said center conductor to
one end of said center conductor differs from distance from a
position where said output line traverses said center conductor to
the other end of said center conductor.
8. The filter according to claim 1, wherein one of said input line
and said output line has a portion that traverses said center
conductor and the other of said input line and said output line has
a portion overlapping said center conductor along a direction in
which said center conductor extends.
9. The filter according to claim 1, wherein said center conductor
has a length of half a wavelength corresponding to a resonance
frequency, a longitudinal middle point of said center conductor
being the position of the null point of the voltage
displacement.
10. The filter according to claim 1 wherein said substrate is a
dielectric substrate.
11. The filter according to claim 1, further comprising a second
substrate arranged so as to cover said coplanar line resonator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high frequency filter
employed in the ultra high frequency band (approximately 1 to 100
GHz) covering the frequency range such as microwave and
millimeter-wave bands, and particularly to an easily designable,
variable-frequency high frequency filter adapted to use a coplanar
line resonator and capable of electronically controlling the filter
characteristics such as a transmission characteristic, particularly
a band characteristic or the like.
[0003] 2. Description of the Related Arts
[0004] A high frequency filter has been widely employed as a
functional element imperative for injection and extraction of a
desired signal and also suppression and elimination of unwanted
signals in transmission and reception apparatuses, in a variety of
radio communication facilities, optical fiber high-speed
transmission apparatuses and measuring instruments related to the
above apparatuses.
[0005] Conventionally, the high frequency filters for the microwave
and higher-frequency bands have been realized typically through the
use of metal waveguides and dielectric resonators. The high
frequency filters of a microwave integrated circuit configuration
as well have been used in recent years for the purpose of promoting
the scaling down of circuitry. The present inventors have proposed
in Japanese Patent Laid-open Publication No. 2003-115701 (JP,
P2003-115701A) a high frequency filter using a coplanar line
resonator, having a microwave integrated circuit configuration and
capable of electronically controlling the filter
characteristics.
[0006] FIG. 1A is a schematic plan view illustrating a conventional
high frequency filter using a coplanar line resonator and capable
of electronically controlling the filter characteristics. FIG. 1B
is a cross-sectional view along line A-A of FIG. 1A.
[0007] This high frequency filter is made up through the use of a
coplanar line, which is a transmission line of a coplanar
configuration, as a resonator. A coplanar configuration refers to a
high frequency transmission line made up of a metal conductor
formed on one principal surface of a substrate. Hence, a
transmission line made of a microstrip line is not included in a
transmission line of a coplanar configuration, because a microstrip
line needs, in addition of a signal line provided on the one
principal surface of the substrate, a ground conductor provided on
the other principal surface of the substrate.
[0008] Ground conductor 2 is provided on one principal surface of
substrate 1 made of dielectric material and a rectangular opening 3
is formed in ground conductor 2. In opening 3, center conductor 4,
which functions as a signal line, is provided extending in the
longitudinal direction of opening 3. The coplanar line resonator is
configured by ground conductor 2 provided on the one principal
surface of substrate 1 and center conductor (i.e., signal line) 4
arranged inside opening 3 formed in ground conductor 2, as
described above. Here, the resonator is constructed such that the
length of center conductor 4 is approximately .lambda./2, wherein
it is assumed that the wavelength corresponding to the intended
resonance frequency f0 is .lambda.. Both ends of center conductor 4
are spaced apart from ground conductor 2 at both ends (the left and
right ends in the figure) of opening 3 thereby forming electrically
open ends. This arrangement allows generation of a standing wave
having a null point (i.e., node) of a voltage displacement at the
middle point that longitudinally bisects center conductor 4 and
maximum (peak) voltage displacements of mutually reverse polarities
at both longitudinal ends of center conductor 4 as depicted as
curve S illustrated in FIG. 1B, yielding a capability of acting as
a resonator. It should be noted that the coplanar line is an
unbalanced transmission line that allows traveling of a high
frequency electromagnetic wave caused by an electric field,
generated between center conductor 4 and ground conductor 2, and a
magnetic field induced by the electric field.
[0009] Furthermore, in the one principal surface of substrate 1,
variable capacitance diodes 5 are arranged individually allocated
to both end portions of opening 3, i.e., the gaps between both ends
of center conductor 4 and ground conductor 2. In the illustrated
example, using solder for example, variable capacitance diodes 5
connect respective ends of center conductor 4 and the edge portions
of ground conductor 2 across the end portions of opening 3 with the
anodes connected to center conductor 4. To the middle point of the
coplanar line resonator, i.e., the middle point that longitudinally
bisects center conductor 4, is connected one end of one of supply
lines 6a for applying control voltage Vc to variable capacitance
diodes 5. The other supply line 6b, i.e., one end of the ground
line, is connected to ground conductor 2. The above arrangement
allows applying control voltage Vc, which is a reverse voltage (a
negative voltage), to the anode of each variable capacitance diode
5 and varying the capacitance values of the diodes.
[0010] Input line 7 and output line 8 are provided on the other
principal surface of substrate 1 in the respective areas
corresponding to both end portions of center conductor 4. Input
line 7 is made up of a closed loop section, which surrounds the
left end portion, as viewed in the figure, of center conductor 4
and an extension section that extends from the closed loop section
to the left end portion, as viewed in the figure, of substrate 1.
The closed loop section of input line 7 is provided to traverse
center conductor 4 under the neighborhood of the left end, as
viewed in the figure, of center conductor 4 and further surround
variable capacitance diode 5. Similarly, output line 8 is made up
of a closed loop section, which surrounds the right end portion, as
viewed in the figure, of center conductor 4 and an extension
section that extends from the closed loop section to the right end
portion, as viewed in the figure, of substrate 1. The closed loop
section of output line 8 is provided to traverse center conductor 4
under the neighborhood of the right end, as viewed in the figure,
of center conductor 4 and further surround variable capacitance
diode 5. These input line 7 and output line 8 form microstrip line
structures together with ground conductor 2 and are electrically
connected with the coplanar line, which acts as a resonator,
through electromagnetic coupling. The position that input line 7 or
output line 8 traverses center conductor 4 is referred to as a
transverse point. In this example, the distance between the
transverse point of input line 7 and the left end of center
conductor 4 is taken to be equal to the distance between the
transverse point of output line 8 and the right end of center
conductor 4. This distance is denoted as d.
[0011] This structure of the resonator allows generating a
plurality of resonance points operable as an input/output resonance
point in the high frequency filter depending on a boundary
condition stipulated on the basis of the positions of input line 7
and output line 8 provided on the other principal surface of the
substrate and traversing the coplanar line resonator, for example,
the lengths from the transverse points of input line 7 and output
line 8 to the ends of center conductor 4. In the above example,
because length d between the transverse point and the end of center
conductor 4 is the same for input line 7 and for output line 8,
basically one input/output resonance point is generated at the
frequency corresponding to a wavelength wherein one fourth the
wavelength equals d. Specifically, because the length from the
transverse point to the tip of the center conductor is one-fourth
the wavelength, both ends of the center conductor behave as
electrically short-circuit ends as viewed from respective
transverse points. Consequently, a high frequency current is
created having one-fourth the wavelength equal to that length,
yielding an input/output resonance point that causes a voltage fall
in the frequency region on the high-frequency side of the resonance
characteristic of center conductor 4. Because center conductor 4
functions as a both-end open half-wavelength resonator, and because
the distances from respective transverse points to the
corresponding ends of center conductor 4 are necessarily shorter
than half the length of center conductor 4 itself, the resonance
frequency at the input/output resonance point is necessarily higher
than the resonance frequency of the coplanar line resonator.
[0012] As shown in FIG. 2, in this high frequency filter,
attenuation pole P due to the input/output resonance point is
created in the frequency region on the high-frequency side of the
band characteristic curve (curve T) for the high frequency filter
provided with the coplanar line resonator. Consequently, the
characteristic curve exhibits a steep gradient of attenuation on
the high frequency side of the resonance frequency of the coplanar
line resonator f0, as shown by curve U. As a result, the band
characteristic of the high frequency filter comes to have a
substantially narrowed-down bandwidth, entailing enhancement of an
apparent Q value. In the above example, the distances d between
both ends of center conductor 4 and the respective transverse
points of input line 7 and output line 8 are equal to each other.
As a result, the two input/output resonance points are
substantially degenerated to one resonance point causing the
attenuation level at the attenuation pole P to increase by just
that much.
[0013] Further, connecting variable capacitance diodes 5 between
both ends of the coplanar line resonator, i.e., both ends of center
conductor 4, and ground conductor 2 allows variation of resonance
frequency f0 to be caused through the variation of the capacitance
by means of control voltage Vc. In this arrangement, because
variable capacitance diodes 5 are arranged in the electric fields
generated between center conductor 4 and ground conductor 2, the
capacitance variation of variable capacitance diodes 5 yields
equivalently the variation of an electrical length of center
conductor 4. In this way, a so-called voltage-controlled high
frequency filter is constituted.
[0014] In the foregoing high frequency filter, employing a coplanar
line resonator of a coplanar structure enables both terminals of
variable capacitance diodes 5 to be connected on the same plane to
apply the surface mount technology to the mounting of variable
capacitance diodes 5. In addition, connecting the supply lines 6a
to the middle point that bisects center conductor 4, i.e.,
connecting the supply lines 6a to the middle point of a
half-wavelength resonator, which is a null point (a minimum point)
of the voltage displacement, and applying control voltage Vc to the
middle point substantially minimizes the influence of the providing
of the supply line on the resonance characteristic.
[0015] The high frequency filter using the coplanar line resonator
of the above structure, however, is configured such that, while
both ends of center conductor 4 are spaced apart from ground
conductor 2 at both ends of opening 3 electrically to make open
ends, variable capacitance diodes 5 are arranged there. Variable
capacitance diode 5 varies the value of its capacitance through the
application of control voltage Vc from supply lines 6 provided in
the middle point of center conductor 4. In this arrangement, for
example, if the value of the capacitance of variable capacitance
diodes 5 is large, then a high frequency current corresponding to
the capacitance of variable capacitance diodes 5 is generated at
both ends of center conductor 4 causing the characteristics of the
resonator to vary in the direction from an ideal electrical open
end to a short-circuit end. As a result, a variation of control
voltage Vc causes changes in positions of the maximum voltage
displacements at both end portions of center conductor 4 and
further causes the null point of the voltage displacement, which
should be in the middle point of center conductor 4, to displace
making the null point deviate from the middle point. Consequently,
the position of one of the supply lines 6a provided at the middle
point of center conductor 4 deviates from the null point of the
voltage displacement, i.e., the position of the supply line 6a
comes to the position where a voltage displacement is present due
to the voltage standing wave. As a result, connection of supply
line 6a affects the resonance characteristics of the resonator,
which makes it difficult to design the resonator. For example, when
the capacitance of variable capacitance diodes 5, to which
reference control voltage Vco is applied, is taken as a reference
capacitance and the central resonance frequency of the resonator
for the reference capacitance is denoted as f0, it becomes
difficult to grasp the variation of resonance frequency when a
control voltage differing by some value from reference control
voltage Vco is applied. In addition, the deviation of the position
of the null point from the middle point of the resonator brings
about difficulty in the foregoing control of the frequency of the
input/output resonance point, further entailing difficulty in
designing a filter having desired attenuation characteristics.
[0016] The arrangement of connecting capacitances created by the
variable capacitance diodes with both ends of center conductor 4
acts to lower the maximum voltage value (the magnitude of the
voltage displacement) in the point of the maximum voltage
displacement of the standing wave induced in center conductor 4.
Furthermore, in the case where the capacitance variation
characteristics of the pair of variable capacitance diodes 5
against control voltage Vc are different, the balance with respect
to the middle point of center conductor 4 is lost, entailing a
loss. Due to the above facts, the Q value, which indicates
resonance sharpness, lowers and the resonance characteristic of the
resonator is degraded.
SUMMARY OF THE INVENTION
[0017] It is an object of the present invention to provide a
frequency-variable high frequency filter configured to have a
position of the voltage-displacement null point in a coplanar line
resonator fixed to facilitate designing the filter and also
configured to obviate lowering the Q value to improve the resonance
characteristics.
[0018] The object of the present invention can be attained by the
high frequency filter comprising: a substrate; a ground conductor
having an opening provided on one principal surface of the
substrate; a center conductor provided on the one principal surface
of the substrate in the opening and making up a coplanar line
resonator of a coplanar structure together with the substrate and
the ground conductor; an input line and an output line provided on
the other principal surface of the substrate and adapted to
electromagnetically couple with the center conductor; and a
variable reactance element inserted in the center conductor at the
position of the null point of the voltage displacement of a
standing wave created in the center conductor operating as a
resonator, wherein the center conductor is divided into a first
conductor section and a second conductor section at the inserted
position of the variable reactance element so as to separate the
first and second conductor sections in the longitudinal direction
of the center conductor, the variable reactance element being
connected at one end to the first conductor section and at the
other end to the second conductor section.
[0019] According to the present invention, it is enabled to make
the resonance frequency of the filter variable through the use of
the control voltage applied to a voltage-controlled variable
reactance element, because the center conductor of the coplanar
line resonator is divided at the null point of the voltage
displacement, for example, at the middle point in the longitudinal
direction of the center conductor, where the variable reactance
element is connected. The influence of the provision of the
variable reactance element and the circuitry to apply a control
voltage to the variable reactance element on the resonance
characteristic can be prevented, because the element and circuitry
are arranged in the neighborhood of the null point of the voltage
displacement in the standing wave. Nor, the control voltage causes
the position of the null point in the voltage displacement to be
displaced. For these reason, the present invention enables easily
grasping the relation between the magnitudes of the variations in
the control voltage and the resonance frequency and facilitating
the designing of the filter and also enables realizing a low loss
and high Q-value filter having a superior resonance
characteristic.
[0020] Furthermore, providing at least one of the input and output
lines so as to traverse the center conductor yields an input/output
resonance point at a frequency point higher than the resonance
frequency of the coplanar line resonator to comply with the
boundary condition, which enables establishing an attenuation pole
on the high frequency side of the resonance frequency, thereby
boosting the attenuation gradient on the high frequency side.
Further, establishing the input/output resonance point makes it
possible to realize a desired filter characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a plan view illustrating a conventional
frequency-variable high frequency filter;
[0022] FIG. 1B is a cross-sectional view taken along line A-A of
FIG. 1A:
[0023] FIG. 2 is a graph illustrating the transmission
characteristic (i.e., filter characteristic) of the high frequency
filter shown in FIGS. 1A and 1B;
[0024] FIG. 3A is a plan view illustrating the high frequency
filter according to the first embodiment of the present
invention;
[0025] FIG. 3B is a cross-sectional view taken along line A-A of
FIG. 3A;
[0026] FIG. 4 is a graph illustrating the transmission
characteristic (i.e., filter characteristic) of the high frequency
filter based on the first embodiment;
[0027] FIG. 5A is a plan view illustrating the high frequency
filter according to the second embodiment of the present
invention;
[0028] FIG. 5B is a cross-sectional view taken along line A-A of
FIG. 5A;
[0029] FIG. 6A is a plan view illustrating the high frequency
filter according to an alternative embodiment of the present
invention; and
[0030] FIG. 6B is a cross-sectional view taken along line A-A of
FIG. 6A.
DETAILED EXPLANATION OF PREFERRED EMBODIMENT
[0031] In FIGS. 3A and 3B that illustrate a high frequency filter
of a first embodiment according to the present invention, the
constituent elements that are Identical to the constituent elements
in FIGS. 1A and 1B bear the same reference numerals, and repeated
explanation of such elements is simplified.
[0032] The high frequency filter illustrated in FIGS. 3A and 3B is
a voltage-controlled high frequency filter of a frequency variable
type, provided with a coplanar line resonator of a coplanar
structure. Specifically, ground conductor 2 is provided on one of
the principal surfaces of substrate 1 made of dielectric material,
and rectangular opening 3 is formed in ground conductor 2. Center
conductor 4 that extends in the longitudinal direction of opening 3
is provided in opening 3. Both ends of center conductor 4 are
spaced apart from ground conductor 2. As with the case of a
conventional high frequency filter shown in FIGS. 1A and 1B, input
line 7 and output line 8 are provided on the other principal
surface of substrate 1, and each of input line 7 and output line 8
has a closed loop section that traverses center conductor 4. Each
of input line 7 and output line 8 makes a microstrip line structure
making use of ground conductor 2, and is formed in the range from
the middle point to the corresponding end of center conductor 4. In
the present embodiment, the distance d from the transverse point to
an end of center conductor 4 is the same for input line 7 and for
output line 8, wherein the transverse point refers to the position
where each of input line 7 and output line 8 traverses center
conductor 4. Center conductor 4 is .lambda./2 long. wherein
.lambda. stands for the wavelength corresponding to resonance
frequency f0 of the filter.
[0033] In the present embodiment, center conductor 4 is divided, at
the position of the longitudinally middle point of center conductor
4, into two sections, which are arranged spaced apart from each
other. The sections of the divided center conductor 4 are
hereinafter referred to as conductor sections 4a and 4b,
respectively. Variable capacitance diode 5 is arranged in the gap
between conductor sections 4a and 4b with the anode and cathode
connected with the opposed ends of conductor sections 4a and 4b
across the gap. Further, variable capacitance diode 5 is connected
with a pair of supply lines 6a, 6b and control voltage Vc is
applied such that the cathode of variable capacitance diode 5 takes
the ground potential and the anode has a reverse voltage (i.e.,
negative voltage).
[0034] This structure yields an input/output resonance point in
compliance with the boundary condition stipulated on the basis of
the positions of input line 7 and output line 8 traversing the
coplanar line resonator, as with the conventional high frequency
filter shown in FIGS. 1A and 1B, entailing creation of an
attenuation pole P on the high frequency side in the band
characteristic curve of the resonator. Because the distances from
the transverse points to the corresponding ends of center conductor
4 are the same for input line 7 and output line 8, the same
boundary condition holds for both of input line 7 and output line
8. This causes degeneration of the input/output resonance points
leading to creation of a single attenuation pole of a high
attenuation level. Consequently, the attenuation gradient becomes
steep in the region of frequencies higher than the resonance
frequency f0, resulting in the reduction of the passband width and
increase in an apparent Q value.
[0035] In this high frequency filter, because variable capacitance
diode 5 is connected between conductor sections 4a and 4b that make
up the coplanar line resonator, the capacitance variation of
variable capacitance diode 5 generated by applied control voltage
Vc equivalently causes an overall electrical length of center
conductor 4 to be varied, thereby changing the resonance frequency
of the filter.
[0036] In this arrangement, both ends of center conductor 4
function as electrical open ends that are completely open-circuited
from ground conductor. 2. Accordingly, even when control voltage Vc
is applied to variable capacitance diode 5 to control the resonance
frequency, the line impedance substantially keeps infinity at each
of both ends of center conductor 4, and both ends of center
conductor 4, which is a half wavelength resonator, become the
maximum voltage displacement points having mutually opposite phases
of the standing wave. The middle point of center conductor 4, i.e.,
the position where conductor sections 4a and 4b face each other
across a gap, acts as a null point of the voltage displacement
regardless of control voltage Vc.
[0037] For this reason, although the anode and cathode of variable
capacitance diode 5 are connected to the opposing ends of conductor
sections 4a and 4b and supply lines 6a and 6b are connected to
these ends, in the neighborhood of the middle point of center
conductor 4, it is possible to suppress the influence of providing
the diode and supply lines on the resonance characteristics and on
the standing wave in the resonator, because the positions where the
diode and supply lines are connected are in the region of the null
point of the voltage displacement regardless of control voltage Vc.
As a result, it becomes feasible to grasp a theoretical variation
of the resonance frequency against control voltage Vc, thereby
enabling the designing of a high frequency filter to be
facilitated. In other words, it is facilitated to grasp the
deviation of the resonance frequency from f0 against control
voltage Vc, wherein f0 stands for the central resonance frequency
when the reference control voltage Vco is applied to variable
capacitance diode 5. While not shown, a choke coil or the like for
blocking high frequency components can be inserted as required in
supply lines 6a and 6b.
[0038] The arrangement of both ends of center conductor 4 spaced
apart from ground conductor 2 to construct electric open ends
allows the maximum voltage displacement points at both ends of
center conductor 4 to preserve the maximum voltages without any
decrease. The standing wave on both sides of the middle point of
the resonator preserves a symmetric property, because, in
principle, a single variable capacitance diode is inserted in the
middle point of center conductor 4. For these reasons, the high
frequency filter of the present embodiment enables suppressing a
loss and boosting the Q value, thereby improving the resonance
characteristics.
[0039] In the above high frequency filter, while it is presumed
that the distances between the transverse points and the
corresponding ends of the center conductor for input line 7 and
output line 8 equal each other and equal d, it is feasible to make
the distance for input line 7 different from that for output line
8. Such an arrangement, however, generates two input/output
resonance points in principle, because different boundary
conditions are applied to input line 7 and output line 8.
Consequently, two attenuation poles Ps are created associated with
the transmission characteristic exclusively of a half-wavelength
resonator (figured curve T) outside the band of the transmission
characteristic as shown in FIG. 4, resulting in creation of a
transmission characteristic of a great attenuation amount in a wide
frequency region (figured curve U).
[0040] It is possible to create resonance points at which
attenuation levels differ depending on a boundary condition other
than the distance between each of input line 7 and output line 8
and the corresponding end of center conductor 4. The number and
positions of these additional input/output resonance points can be
determined depending on the specifications of the high frequency
filter, i.e., the required transmission characteristics and
selected as required.
[0041] Explanation is next presented for the high frequency fitter
according to a second embodiment of the present invention. The high
frequency filter of the second embodiment illustrated in FIGS. 5A
and 5B is configured such that a change is made in the structure of
the input line in the high frequency filter of the first embodiment
shown in FIGS. 3A and 3B. Specifically, while in the high frequency
filter of the first embodiment, both of input line 7 and output
line 8 have closed loop sections each of which traverses center
conductor 4, input line 7 of the high frequency filter of the
second embodiment has no closed loop section. Output line 8 has a
closed loop section.
[0042] More specifically, in the high frequency filter of the
second embodiment, center conductor 4 has conductor sections 4a and
4b made by dividing center conductor 4 at the longitudinally middle
point of center conductor 4 with variable capacitance diode 5
arranged between and connected across conductor sections 4a and 4b,
just like the high frequency filter of the first embodiment. In
addition, a pair of supply lines 6a and 6b are provided to apply
control voltage Vc.
[0043] Input line 7, which is provided on the other principal
surface of substrate 1 and functions as a microstrip line, linearly
extends from the left side in the figure along the extending
direction of center conductor 4 with the tip position reaching the
middle area of conductor section 4a. Accordingly, input line 7 is
formed in an overlapped fashion with center conductor 4 at the
neighborhood of its tip position along the same direction as center
conductor 4, thereby electrically connected with conductor section
4a basically through the capacity coupling. Output line 8 has a
closed loop section and traverses conductor section 4b, as with the
first embodiment.
[0044] This arrangement creates input/output resonance point at a
resonance frequency higher than that of the coplanar line resonator
depending on the boundary condition based on the position of output
line 8 as described in the first embodiment. This input/output
resonance point creates attenuation pole P on the high frequency
side of the band characteristic of the filter having a steep
attenuation gradient. Input line 7, in contrast, creates no
input/output resonance point attributed to the boundary condition,
because input line 7 does not traverse but only overlaps with one
of the divided conductors 4a in the same direction. In this filter,
since it suffices for the input/output resonance point basically to
take only one resonance point caused by output line 8 into account,
the number of the input/output resonance points is fewer than that
in the first embodiment, which facilitates designing.
[0045] In this high frequency filter, since basically a single
variable capacitance diode 5 is inserted in the middle point of
center conductor 4 of the coplanar line resonator as with the first
embodiment, the neighborhood of the middle point of center
conductor 4 becomes a null area of the voltage displacement
regardless of control voltage Vc applied to the diode. As a result,
it is enabled to reduce an influence of providing a diode and
supply lines on the resonance characteristics and thereby to
facilitate designing of the filter. In addition, the values of the
maximum voltage at the maximum voltage displacement points disposed
at both ends of center conductor 4 are preserved and also the
symmetry of the standing wave is ensured, whereby a loss is
reduced, the Q-value is enhanced and the satisfactory resonance
characteristics are obtained.
[0046] The foregoing explanation regards the high frequency filters
according to the present invention. The high frequency filter of
the present invention is not necessarily limited to the above
filters.
[0047] FIGS. 6A and 6B represents a high frequency filter provided
with an additional dielectric substrate 10 on the one principal
surface of dielectric substrate 1, on which the coplanar line
resonator is formed, so as to cover the coplanar line resonator.
The substrate 10 has through-hole 9 formed in a dimension so as to
allow receiving variable capacitance diode 5. The substrate 10 is
arranged on substrate 1 so that variable capacitance diode 5 may be
exposed in this through-hole 9. In this arrangement, control
voltage Vc can be applied to the anode and cathode of variable
capacitance diode 5 through via-holes 11 provided in substrate
10.
[0048] This arrangement makes it enabled to form a conductor
pattern (not shown) connected to the via-holes 11 on substrate 10,
extend it to the end portion of substrate 10 and connect the
conductor pattern directly to the connectors of a cable for
feeding. Furtherrnore, in this case, it is possible to make the
high frequency filter have further multiple functions by arranging,
for example, alternative circuit elements on the surface of
substrate 10.
[0049] It is also possible to constitute the high frequency filters
of the foregoing present embodiments in a cascaded structure or
multistage structure. In this case, a plurality of coplanar line
resonators are formed along the longitudinal direction of the given
reference resonator on one principal surface of the same dielectric
substrate and coupling lines having closed loops in both ends are
provided on the other principal surface of the dielectric
substrate. Each of the coupling lines acts, at one of the looped
ends, as an output line of the preceding coplanar line resonator
and at the same time, acts, at the other looped end, as an input
line of the subsequent coplanar line resonator. The preceding and
subsequent coplanar line resonators are electromagnetically
interconnected through the coupling line. By cascade-connecting a
plurality of coplanar line resonators in this way, a cascaded
variable frequency high frequency filter can be realized. In this
case also, due to the coupling line connecting respective coplanar
resonators, each of the coplanar line resonators creates
input/output resonance points at the frequencies higher than the
resonance frequency of the resonator itself. As a result,
attenuation poles P are created on the high frequency side of the
band characteristic of each coplanar line resonator, causing the
attenuation gradient on the high frequency side of the band
characteristic curve to be steepened. If the central resonance
frequencies of the plurality of the coplanar line resonators are
made coincident with one another, then a sharp band characteristic
can be obtained as a whole, and if the central resonance
frequencies of the plurality of the coplanar line resonators are
staggered, then a wide-band filter characteristic can be
obtained.
[0050] In the above high frequency filter, while the length of the
center conductor is designed to be half a wavelength corresponding
to the resonance frequency, the length can equal one wavelength.
Generally speaking, an integer multiple of a half-wavelength
suffices for the length of the center conductor 4 so that the
voltage distribution of the standing wave will be asymmetric with
respect to the middle point of center conductor 4. While in these
cases, the occasions can take place in which the geometrical middle
point of center conductor 4 viewed in the longitudinal direction is
not the null point of the voltage displacement, it is preferred in
such occasions to divide center conductor 4 at the null point of
the voltage displacement of the standing wave induced in center
conductor 4 and insert a variable reactance element at the divided
position.
[0051] It is feasible as substrate 1 to use, alternatively of the
substrate made of simply dielectric material, a substrate made of
magnetic material or a substrate of semiconductor material.
[0052] Although the above embodiment employs a variable capacitance
diode as a variable reactance element, the present invention does
not limit the variable reactance element to the variable
capacitance diode. In the present invention, any variable reactance
element can be employed if the reactance of the element can be
varied by an applied voltage. For example, it is feasible to use an
element having an inductance variable depending on a control
voltage.
[0053] According to the present invention, because the coplanar
line resonator that makes up a filter has a coplanar structure, not
only a variable reactance element of a surface mount structure but
a beam lead semiconductor element and a flip-chip IC through the
bump mounting or the like can also be mounted on the filter with a
high accuracy and efficiently. The surface mount structure referred
herein refers to the structure that can be arranged on the same
plane and includes the structures having a mounting terminal for
mounting directly on the container body of the element and also
having a lead wire.
* * * * *